1. Field of the Invention
This invention relates generally to flywheel energy storage systems, and more particularly to a flywheel energy storage system that includes a high-speed flywheel assembly, and a plurality of pumps arranged in parallel for reducing windage losses due to gases contained in or evolving from the high-speed flywheel assembly and other portions of the flywheel system.
2. Background
FIG. 1 shows a simplified view of a conventional flywheel energy storage system 100 used for storing kinetic energy. The conventional flywheel system 100 includes a flywheel assembly 104 disposed in a flywheel housing 102. Further, a drag pump 106 is incorporated into the flywheel assembly 104 for pumping gases from the flywheel housing 102 into a separate gas storage chamber 108. For example, FIG. 1 includes arrows for indicating a direction of gas flow from the flywheel housing 102, through helical grooves (not numbered) formed in the drag pump 106, and then into the gas storage chamber 108.
Traditionally, flywheel assemblies have been made of metal, e.g., high strength steel. More recently, flywheel assemblies have been fabricated using fiber composite materials, e.g., fiberglass or carbon fibers wound with a resin binder, thereby making flywheel assemblies that are lighter in weight and capable of operating at higher speeds than the traditional metal flywheel assemblies operate. Both the flywheel assemblies that are made of metal and those made of fiber composite materials typically evolve substantial quantities of gases during operation, thereby potentially increasing gas pressure levels inside flywheel housings to unacceptable levels. Such increased pressures can significantly reduce the useful lifetime of flywheel energy storage systems because they generally lead to high windage losses.
For this reason, pumps like the drag pump 106 shown in FIG. 1 have been used for drawing off evolved gases from flywheel housings. Pumps suitable for this purpose include both turbo-molecular pumps and molecular drag pumps. However, such pumps have drawbacks in that they are typically not designed for pumping evolved gases directly from flywheel housings to the atmosphere.
A common solution to this problem is to provide a mechanical roughing pump at the outlet of a drag pump in a flywheel system. Such mechanical roughing pumps are generally capable of exhausting directly to the atmosphere. As a result, the drag pump and the roughing pump may be used in combination for drawing off the evolved gases in the flywheel housing, thereby reducing gas pressure levels in the flywheel housing for optimal flywheel operation. However, mechanical roughing pumps also have drawbacks, in that they are usually high in cost and typically require frequent maintenance.
Another solution is to provide a gas storage chamber such as the chamber 108 (see FIG. 1) at the outlet of the drag pump. For example, in U.S. Pat. No. 5,462,402 (“the '402 patent”) issued Oct. 31, 1995, to Bakholdin et al., a flywheel energy storage system with an integral molecular pump is disclosed. In accordance with that disclosure, a flywheel assembly used for mobile energy storage incorporates a molecular pump and an internal chamber containing molecular sieves. The molecular pump shares the shaft, bearings, and motor of the flywheel rotor, and maintains the high vacuum desired in the vicinity of the flywheel rotor. The gases, which evolve from all parts within the flywheel system during its operational life, are pumped into the chamber containing molecular sieves, where they are adsorbed.
However, the flywheel energy storage system described in the '402 patent also has some drawbacks. For example, the molecular sieves contained in the internal chamber typically cannot adsorb all of the different types of gases that can evolve during high-speed operation of the flywheel assembly.
Specifically, the evolved gases may include water vapor along with various quantities of hydrocarbons and/or other active gases such as hydrogen or nitrogen. As used herein “active gases” means gases other than water vapor and inert gases. Although molecular sieves can, in general, efficiently adsorb, e.g., water vapor, they typically cannot adsorb substantial quantities of hydrocarbons and/or other active gases, especially at temperatures of about 20° C. and above. This is a significant problem because flywheel assemblies operating at high-speed, especially those made of fiber composite materials, are likely to evolve substantial quantities of active gases. If these gases are not adsorbed by the molecular sieves or otherwise pumped out to the atmosphere, the flywheel system, e.g., the flywheel housing and/or the above-described internal chamber, will likely be subjected to unacceptable gas pressure levels over time, thereby increasing windage losses and significantly limiting the useful lifetime of the flywheel system.
Further, in accordance with the disclosure of the '402 patent, getter materials may be disposed throughout the vacuum housing of the flywheel to absorb trace quantities of gases that are not readily adsorbed by the molecular sieves contained in the internal chamber of the flywheel system.
However, this approach also has some drawbacks. Specifically, as the getter material disposed in the flywheel housing increasingly absorbs the trace quantities of gases, its capacity for further absorbing gases typically degrades. Getter pumps designed for use in flywheel systems typically have limited pumping capacities. As a result, gas pressure surrounding the getter material in the flywheel housing can increase over time, thereby increasing overall gas pressure in the flywheel housing to unacceptable levels.
One way of achieving increased pumping capacity in flywheel systems is to use non-evaporable getter (NEG) pumps, which generally have pumping capacities that are significantly greater than that of evaporated getter pumps. Such NEG pumps typically achieve a maximum capacity for pumping various gases at elevated temperatures, e.g., 250° C. or higher. For example, in U.S. Pat. No. 5,879,134 (“'134”) issued Mar. 9, 1999, to Lorimer et al., a getter pump for pumping gases in a wafer processing system is disclosed. In accordance with that disclosure, a wafer processing system includes a processing chamber, a low-pressure pump coupled to the processing chamber for pumping gases, a valve mechanism coupling a source of inert gas to the processing chamber, an in situ getter pump disposed within the processing chamber which pumps certain active gases during the flow of the inert gas into the chamber, and a processing mechanism for processing a wafer disposed within the processing chamber. Preferably, the in situ getter pump can be operated at a number of different temperatures to preferentially pump different species of gas at those temperatures. A gas analyzer is used to automatically control the temperature of the getter pump to control the species of gases that are pumped from the chamber.
However, systems incorporating the getter pumps for pumping gases as described in the '134 patent typically consume significant amounts of power. Although high power consumption might be acceptable in systems such as wafer processing systems, it is generally unacceptable in flywheel energy storage systems.
In addition, as explained above, gases that evolve from flywheel systems typically include water vapor along with lesser quantities of hydrocarbons and/or other active gases. Further, the getter material disposed in the flywheel housing is usually capable of absorbing all of these evolved gases inside the housing, thereby rapidly and significantly degrading the capacity of the getter material for further absorbing gases. This not only causes gas pressure levels of the evolved gases to increase over time, but also significantly increases costs because such getter materials used with flywheel systems are relatively expensive.
Restricting gas flow to the getter material can significantly reduce the speed at which the getter material degrades, thereby reducing the cost of using the getter material. For example, in U.S. Pat. No. 4,272,259 (“the '259 patent”) issued Jun. 9, 1981, to Patterson et al., a gas gettering system is disclosed. In accordance with that disclosure, a fluid-tight container holding active getter and non-sorbable gas at a pressure of at least about one atmosphere is provided, with gas flow passage means through a wall of the container providing communication between the active getter and the container-surrounding environment and removable closure means for the gas flow passage means. The container may be opened for gas flow communication and exposure of the active getter prior to sealing of a vacuum enclosure in which it is installed, without significant loss or impairment of sorptive capacity of the getter.
However, the gas gettering system described in the '259 patent also has some drawbacks. For example, restricting gas flow to the getter material not only reduces the speed at which the getter material degrades, but also reduces the speed at which the getter material pumps gases. This can be problematic in flywheel systems because if the evolved gases are not pumped out of the flywheel housing at a fast enough rate, gas pressure levels inside the flywheel housing will likely rise to unacceptable levels, thereby increasing windage losses and reducing the useful lifetime of the flywheel system.
Providing a plurality of getter materials can also reduce the speed at which getter materials degrade, thereby reducing costs. For example, in U.S. Pat. No. 4,297,082 (“the '082 patent”) issued Oct. 27, 1981, to Wurtz et al., a vacuum gettering arrangement is disclosed. In accordance with that disclosure, the vacuum gettering system includes first bulk getter of zirconium-aluminum alloy and having a heater therein for activation. Second bulk getter of porous silica glass is directly adjacent to the first bulk getter for heating activation. As the vacuum enclosure is pumped out, the heater heats both getters to activation temperature to drive off gases and vapors during low temperature enclosure baking and pump-out so that at enclosure close-off both getters are fully activated.
Again, the vacuum gettering arrangement described in the '082 patent has some drawbacks. For example, that vacuum gettering system includes a heater for heating both the first bulk getter and the second bulk getter to activation temperature. As explained above, such getter pumps that require heat activation often consume significant amounts of power, which is generally unacceptable in flywheel energy storage systems.
Further, the '082 patent discloses that the first bulk getter of zirconium-aluminum alloy is principally for light gas absorption, while the second bulk getter of porous silica glass is principally for water absorption. However, even though the first and second bulk getter materials are meant to be used for absorbing specific types of gases, in practice getter materials are frequently capable of absorbing different types of gases, including water vapor. As a result, the relatively expensive getter material used for absorbing trace gases might still be quickly degraded by absorbing substantial quantities of, e.g., water vapor, along with the trace gases. Such a gettering arrangement would be unsuitable for use in low-power, low-cost, flywheel energy storage systems.
It would therefore be desirable to have a flywheel energy storage system that has lower windage losses and a longer useful life than conventional flywheel energy storage systems. Such a flywheel energy storage system would be capable of successfully drawing off gases that typically evolve from a flywheel system during operation, thereby creating a near-vacuum in the flywheel housing for optimal flywheel operation. It would also be desirable to have such vacuum pumping in a low-power, low-cost, flywheel energy storage system.